1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive comprising a n-current preamplifier for generating n-independently controlled write currents for a plurality of heads.
2. Description of the Prior Art
Magnetic disk drives for computer systems typically employ an array of disks and associated read/write heads together with head positioning and spindle mechanics. This arrangement of heads and fixed disk array is referred to as a head disk assembly or HDA, an overview of which is provided in FIG. 1A. Several magnetic disks 2 connected in an array are rotated by a spindle motor. Each recording surface (top and bottom) of each magnetic disk is accessed through a dedicated head 4; as the disks spin, a thin layer air-bearing forms between the heads 4 and the recording surface such that the heads 4 are said to "fly" just above the recording surface. The heads 4 are connected to the distal end of actuator arms 6 which are connected to a pivot 8 actuated by a rotary voice coil motor (VCM). As the VCM rotates the actuator arms 6 about the pivot 8, the heads 4 are positioned radially over the recording surface so that information can be written to and read from the recording surface.
The recording surface of the magnetic disk is coated with a thin film medium (e.g., cobalt alloy) which is magnetized inductively by a write coil of the head 4. The digital data being recorded modulates a current passing through the write coil in order to inductively write a series of magnetic transitions onto the disk surface (recording surface) of the disk, where a preamplifier chip incorporated within the HDA performs the modulation function in response to the digital data. As shown in FIG. 1B, the data is written in the radially spaced, concentric tracks 10 which are partitioned into blocks of data referred to as data sectors 12. Because the circumferential recording area increases from the inner to outer diameter tracks, more data can be stored in the outer diameter tracks. Thus, in order to maintain a more constant linear bit density and thereby maximize the overall storage capacity, the recording surface is normally partitioned into a number of zones where each zone comprises a predetermined number of tracks. Data is then written to the recording surface at an increasing rate as the head traverses radially from the inner to outer diameter zones, thereby increasing the amount of data stored in the outer diameter tracks. This is illustrated in FIG. 1B which shows a disk partitioned into an inner diameter zone 14 comprising seven data sectors per track, and an outer diameter zone 16 comprising fourteen data sectors per track. In practice, the recording surface is actually partitioned into several zones with the data rate incrementally increasing from the inner to outer diameter zones in order to exploit the maximum storage capacity of the recording surface.
Typically the magnetic disks 2 also comprise servo sectors 18 which are recorded at a regular interval and interleaved with the data sectors 12 as shown in FIG. 1B. A servo sector, as shown in FIG. 1C, typically comprises a preamble 20 and sync mark 22 for synchronizing to the servo sector; a servo data field 24 comprising coarse position information, such as a Gray coded track address, used to determine the radial location of the head with respect to the plurality of tracks; and a plurality of servo bursts 26 recorded at precise intervals and offsets from the track centerlines which provide fine head position information. When writing or reading data, a servo controller performs a "seek" operation to position the head over a desired track; as the head traverses radially over the recording surface, the Gray coded track addresses in the servo data field 24 provide coarse position information for the head with respect to the current and target track. When the head 4 reaches the target track, the servo controller performs a tracking operation wherein the servo bursts 26 provide fine position information used to maintain the head over the centerline of the track as the digital data is being written to or read from the recording surface.
The servo sectors 18 are written to the recording surfaces as part of the manufacturing process to enable the seek and tracking operations necessary to write and read the data sectors 12. A common mechanism for writing the servo sectors to the recording surfaces is an external servo track writer which uses the write preamplifier electronics and heads within the HDA, but which uses separate control circuitry and servo mechanics for radially positioning the heads using well known techniques such as a laser interferometer. A significant cost reduction can be achieved by a "self-servowriting" method which can use circuitry in the disk drive for writing the servo sectors.
It is desirable to expedite the process of writing the servo sectors 18 to the array of recording surfaces within each disk drive to maximize manufacturing throughput. It is known to write the servo sectors 18 to all of the recording surfaces simultaneously by using a technique referred to as "bank servo writing" wherein the write current generated by the preamplifier is applied to all of the heads to simultaneously write the servo sectors to all of the recording surfaces rather than one surface at a time. This is illustrated by the prior art preamplifier shown in FIG. 4 wherein a register 28 is loaded with a digital write current setting converted into an analog write current setting 30 by a digital-to-analog converter (DAC) 32. The analog write current setting 30 adjusts the output current of driver circuits (34.sub.0 -34.sub.N) which supply the respective write currents (36.sub.0 -36.sub.N) to the heads 4. Head select circuitry 38 within the preamplifier enables the output of the appropriate driver circuit (34.sub.0 -34.sub.N) over line 40 during normal operation of the disk drive, and it enables the output of all the driver circuits (34.sub.0 -34.sub.N) during servo track writing in order to write the servo sectors to all of the recording surfaces simultaneously. The digital write data 42 to be recorded to the surface of the disk 2 modulates the operation of the driver circuits (34.sub.0 -34.sub.N) by alternating the polarity of the write current 36; for example, a digital "1" bit may modulate a positive write current and a digital "0" may modulate a negative write current.
Noise in the disk drive (electronic noise, media noise, intersymbol interference, etc.) may induce errors when reading the track addresses and/or servo bursts which will degrade the performance of the disk drive by increasing seek times as well as increasing the bit error rate if the head is unable to maintain proper centerline tracking. Therefore, when the servo sectors are written to the recording surfaces, it is important that enough write current is supplied to each head to saturate the magnetic material on the recording surface so as to maximize the signal power during read back. Prior art servo track writers that perform a bank servo write to all of the recording surfaces simultaneously would set the write current high enough to ensure that each head would be driven by enough current to saturate the recording surfaces. Setting the write current higher than the minimum required to saturate the recording surface does not significantly reduce the signal-to-noise ratio when using a conventional inductive head which comprises a single coil for both writing and reading the magnetic transitions. This is because the poles in a conventional inductive head are essentially the same width which results in minimal fringing fields emanating from the periphery of the write gap even if the write current is set higher than necessary. This is not the case, however, with magneto-resistive (MR) heads which comprise an inductive write element (write coil) and a MR read element integrated into one head. In typical MR heads having two poles, one pole of the inductive write element is shared with one of the shields for the MR read element; this pole is consequently wider than the other pole of the inductive write element which causes significant fringing fields at the periphery of the write gap if the write current is set too high. Further, the amount of write current necessary to saturate the recording surface varies between the MR heads in the disk array due to process variations in manufacturing the MR heads and the magnetic disks. Thus, using the prior art preamplifier of FIG. 4 to drive all the MR heads with a single write current high enough to ensure that all of the recording surfaces are saturated may inevitably drive at least one of the MR heads with too much write current and cause significant fringing fields.
The fringing fields, if strong enough, will effectively erase an area of the disk at the periphery of the write gap thereby forming an "erase band" along the edges of the servo sector data as well as the servo bursts. This is illustrated in FIG. 2A which shows the two write poles of an MR head, where the second write pole is shared with a shield of the MR read element and therefore is wider than the first write pole. The view of the MR head in FIG. 2A is looking up from the disk with the direction of the MR head and orientation of the track vertical to the page. In addition to the flux lines generated in the write gap between the two poles of the inductive write coil, flinging fields are generated at the periphery of the write gap due to the disparate pole widths. As illustrated, the flinging fields arc from the write pole forming flux lines perpendicular to the track which can effectively erase the recording surface. The width of the adverse flinging fields extends to the critical flux line, the flux line strong enough to change the magnetization of the recording surface, which is proportional to the strength of the write current. In FIG. 2A, the write current is too high causing wide erase bands at the edges of the track. A more optimal write current is shown in FIG. 2B which is just strong enough to generate flux in the write gap to saturate the recording surface along the track, while creating only a narrow erase band due to the attenuated fringing fields.
The magnetic transitions in the servo track addresses are recorded using a phase coherent Gray code meaning that the magnetic transitions in the track addresses of adjacent tracks differ by only two adjacent bit cells so that there is no intertrack interference when the head is between tracks during a seek operation. An erase band caused by the flinging fields of an MR head interferes with the accurate detection of the track addresses by disrupting the phase coherent nature of the Gray code. In addition, the erase band at the edges of the servo bursts introduces a non-linear distortion in the position error signal generated during tracking which offsets the centerline position of the head preventing optimal detection of the data sectors.
Furthermore, the characteristics of the storage medium may change from the inner diameter tracks to the outer diameter tracks such that more or less write current may be necessary to saturate the recording surface depending on the head's radial location. In addition, since the head traverses in an arc trajectory, the characteristics of the erase band formed by the fringing field may vary depending on the radial location of the head. As the head traverses radially over the disk, the poles of the write element will skew from the track centerline depending on the head's arc trajectory, which changes the characteristic of the erase band.
There is, therefore, the need to determine the optimal write current while servo track writing a magnetic disk to ensure that the recording surface is saturated while avoiding erase bands caused by fringing fields when the write current is set too high. Further, there is a need to simultaneously write the servo sectors to a plurality of recording surfaces in a disk array (bank servo write) so as to maximize the manufacturing throughput. In addition, there is a need to optimize the write current with respect to the radial location of the head to compensate for the varying characteristics of the magnetic media as well as the varying characteristics of the erase bands as the head skews from the track centerline.